In lithography for device manufacture, there is an ongoing desire to reduce the size of features in a lithographic pattern in order to increase the density of features on a given substrate area. Patterns of smaller features having critical dimensions (CD) at nano-scale allow for greater concentrations of device or circuit structures, yielding potential improvements in size reduction and manufacturing costs for electronic and other devices. In projection photolithography, the push for smaller features has resulted in the development of technologies such as immersion lithography and extreme ultraviolet (EUV) lithography.
An alternative, so-called imprint lithography, generally involves the use of a “stamp” (often referred to as an imprint template) to transfer a pattern onto a substrate. An advantage of imprint lithography is that the resolution of the features is not limited by, for example, the emission wavelength of a radiation source or the numerical aperture of a projection system. Instead, the resolution is mainly limited to the pattern density on the imprint template.
Block Copolymer Self-Assembly
For both projection photolithography and for imprint lithography, it is desirable to provide high resolution patterning of surfaces, for example of a device substrate, of an imprint template or of other substrates. The use of self-assembly of a block copolymer (BCP) has been considered as a potential method for increasing the feature resolution to a smaller dimension than that obtainable by prior lithography methods or as an alternative to prior lithography methods, such as electron beam lithography for preparation of device substrates or imprint templates.
A self-assemblable BCP is a compound useful in nanofabrication because it may undergo an order-disorder transition on cooling below a certain temperature (order-disorder transition temperature To/d) resulting in phase separation of copolymer blocks of different chemical nature to form ordered, chemically distinct domains with dimensions of tens of nanometers or even less than 10 nm. The size and shape of the domains may be controlled by manipulating the molecular weight and composition of the different block types of the copolymer. The interfaces between the domains may have a line width roughness of the order of about 1-5 nm and may be manipulated by modification of the chemical compositions of the blocks of the copolymer.
The feasibility of using a thin film of BCP as a self-assembling template for pattern formation was demonstrated by Chaikin and Register, et al., Science 276, 1401 (1997). Dense arrays of dots and holes with dimensions of 20 nm were transferred from a thin film of poly(styrene-block-isoprene) to a silicon nitride substrate.
A BCP comprises different blocks, each typically comprising one or more identical monomers, and arranged side-by side along the polymer chain. Each block may contain many monomers of its respective type. So, for instance, an A-B BCP may have a plurality of type A monomers in the (or each) A block and a plurality of type B monomers in the (or each) B block. An example of a suitable BCP is, for instance, a polymer having covalently linked blocks of polystyrene (PS) monomer (hydrophobic block) and polymethylmethacrylate (PMMA) monomer (hydrophilic block). Other BCPs with blocks of differing hydrophobicity/hydrophilicity may be useful. For instance a tri-block copolymer such as (A-B-C) BCP may be useful, as may an alternating or periodic BCP e.g. [-A-B-A-B-A-B-]n or [-A-B-C-A-B-C]m where n and m are integers. The blocks may be connected to each other by covalent links in a linear or branched fashion (e.g., a star or branched configuration).
A BCP may form many different phases upon self-assembly, dependent upon the volume fractions of the blocks, degree of polymerization within each block type (i.e. number of monomers of each respective type within each respective block), and the optional use of a solvent and surface interactions. When applied in a thin film, geometric confinement may pose additional boundary conditions that may limit the phases formed. In general, spherical (e.g. cubic), cylindrical (e.g. tetragonal or hexagonal) and lamellar phases (i.e. self-assembled phases with cubic, hexagonal or lamellar space-filling symmetry) are practically observed in thin films of self-assembled BCPs.
The phase type observed may depend upon the relative molecular volume fractions of the different polymer blocks. For instance, a molecular volume ratio of 80:20 will provide a cubic phase of discontinuous spherical domains of the low volume block arranged in a continuous domain of the higher volume block. As the volume ratio reduces to 70:30, a cylindrical phase will be formed with the discontinuous domains being cylinders of the lower volume block. At a 50:50 ratio, a lamellar phase is formed. With a ratio of 30:70 an inverted cylindrical phase may be formed and at a ratio of 20:80, an inverted cubic phase may be formed.
Suitable BCPs for use as a self-assemblable polymer include, but are not limited to, poly(styrene-b-methylmethacrylate), poly(styrene-b-2-vinylpyridone), poly(styrene-b-butadiene), poly(styrene-b-ferrocenyldimethylsilane), poly(styrene-b-ethyleneoxide), poly(ethyleneoxide-b-isoprene). The symbol “b” signifies “block”. Although these are di-block copolymer examples, it will be apparent that self-assembly may also employ a tri-block, tetra-block or other multi-block copolymer.
One method used to guide or direct self-assembly of a polymer (such as a BCP) onto a substrate surface is known as graphoepitaxy. This method involves the self-organization of a BCP guided by topological pre-patterning on the substrate using one or more features constructed of resist (or one or more features transferred from resist onto a substrate surface, or one or more features transferred onto a film stack deposited on the substrate surface). The pre-patterning is used to form an enclosure or “recess” comprising a substrate base and a sidewall, e.g., a pair of opposing side-walls, of resist (or a side-wall formed in a film or a side-wall formed in the substrate).
Typically, the height of a feature of a graphoepitaxy guiding template is of the order of the thickness of the BCP layer to be ordered, so may be, for instance, from about 20 nm to about 150 nm.
A lamellar self-assembled BCP can form a parallel linear pattern of lithography features with adjacent lines of the different polymer block domains in the recess. For instance if the BCP is a di-block copolymer with A and B blocks within the polymer chain, the BCP may self-assemble into an ordered layer in each recess, the layer comprising regularly spaced first domains of A blocks, alternating with second domains of B blocks.
Similarly, a cylindrical self-assembled BCP can form an ordered pattern of lithography features comprising cylindrical discontinuous first domains surrounded by a second continuous domain. For instance, if the BCP is a di-block copolymer with A and B blocks within the polymer chain, the A block may assemble into a cylindrical discontinuous domain within a circular recess and surrounded by a continuous domain of B block. Alternatively, the A block may assemble into cylindrical discontinuous domains regularly spaced across a linear recess and surrounded by a continuous domain of B block.
Graphoepitaxy may be used, therefore, to guide the self-organization of lamellar or cylindrical phases such that the BCP pattern subdivides the spacing of the side wall(s) of a recess into domains of discrete copolymer patterns.
In a process to implement the use of BCP self-assembly in nanofabrication, a substrate may be modified with a neutral orientation control layer, as part of the graphoepitaxy guiding template, to induce the preferred orientation of the self-assembly pattern in relation to the substrate. For some BCPs used in self-assemblable polymer layers, there may be a preferential interaction between one of the blocks and the substrate surface that may result in orientation. For instance, for a polystyrene(PS)-b-PMMA BCP, the PMMA block will preferentially wet (i.e. have a high chemical affinity with) an oxide surface and this may be used to induce the self-assembled pattern to lie oriented substantially parallel to the plane of the surface. Substantially normal orientation may be induced, for instance, by depositing a neutral orientation layer onto the surface rendering the substrate surface neutral to both blocks, in other words the neutral orientation layer has a similar chemical affinity for each block, such that both blocks wet the neutral orientation layer at the surface in a similar manner. By “normal orientation” it is meant that the domains of each block will be positioned side-by-side at the substrate surface, with the interfacial regions between adjacent domains of different blocks lying substantially perpendicular to the plane of the surface.
In a graphoepitaxy guiding template for aligning a di-block copolymer having A and B blocks, where A is hydrophilic and B is hydrophobic in nature, the graphoepitaxy pattern may comprise hydrophobic side-wall features, with a neutral orientation base between the hydrophobic features. The B domain may preferentially assemble alongside the hydrophobic features, with several alternating domains of A and B blocks aligned over the neutral orientation region between the pinning features of the graphoepitaxy guiding template.
A neutral orientation layer may, for instance, be created by use of random copolymer brushes which are covalently linked to the substrate by reaction of a hydroxyl terminal group, or some other reactive end group, to oxide at the substrate surface. In other arrangements for neutral orientation layer formation, a crosslinkable random copolymer or an appropriate silane (i.e. molecules with a substituted reactive silane, such as a (tri)chlorosilane or (tri)methoxysilane, also known as silyl, end group) may be used to render a surface neutral by acting as an intermediate layer between the substrate surface and the layer of self-assemblable polymer. Such a silane based neutral orientation layer will typically be present as a monolayer whereas a crosslinkable polymer is typically not present as a monolayer and may have a layer thickness of typically less than or equal to about 40 nm, or less than or equal to about 20 nm.
A thin layer of self-assemblable BCP may be deposited onto a substrate having a graphoepitaxy guiding template as set out above. A suitable method for deposition of the self-assemblable polymer is spin-coating, as this process is capable of providing a well-defined, uniform, thin layer of self-assemblable polymer. A suitable layer thickness for a deposited self-assemblable polymer film is approximately 10 nm to 150 nm.
Following deposition of the BCP film, the film may still be disordered or only partially ordered and one or more additional steps may be needed to promote and/or complete self-assembly. For instance, the self-assemblable polymer may be deposited as a solution in a solvent, with solvent removal, for instance by evaporation, prior to self-assembly.
Self-assembly of a BCP is a process where the assembly of many small components (the BCP) results in the formation of a larger more complex structure (the nanometer sized features in the self-assembled pattern). Defects arise naturally from the physics controlling the self-assembly of the polymer. Self-assembly is driven by the differences in interactions (i.e. differences in mutual chemical affinity) between A/A, B/B and A/B (or B/A) block pairs of an A-B BCP, with the driving force for phase separation described by Flory-Huggins theory for the system under consideration. The use of graphoepitaxy may greatly reduce defect formation. The Flory-Huggins interaction parameter (chi value), and the degree of polymerization of the BCP blocks (N value) are parameters of the BCP which affect the phase separation, and the dimensions with which self-assembly of a particular BCP occurs.
For a polymer which undergoes self-assembly, the self-assemblable polymer will exhibit an order-disorder temperature To/d. To/d may be measured by any suitable technique for assessing the ordered/disordered state of the polymer, such as differential scanning calorimetry (DSC). If layer formation takes place below this temperature, the molecules will be driven to self-assemble. Above the temperature To/d, a disordered layer will be formed with the entropy contribution from disordered A/B domains outweighing the enthalpy contribution arising from favorable interactions between neighboring A-A and B-B block pairs in the layer. The self-assemblable polymer may also exhibit a glass transition temperature Tg below which the polymer is effectively immobilized and above which the copolymer molecules may still reorient within a layer relative to neighboring copolymer molecules. The glass transition temperature is suitably measured by differential scanning calorimetry (DSC).
Defects formed during ordering as set out above may be partly removed by annealing. A defect such as a disclination (which is a line defect in which rotational symmetry is violated, e.g. where there is a defect in the orientation of a director) may be annihilated by pairing with other another defect or disclination of opposite sign. Chain mobility of the self-assemblable polymer may be a factor for determining defect migration and annihilation and so annealing may be carried out at a temperature where chain mobility is high but the self-assembled ordered pattern is not lost. This implies temperatures up to a few ° C. above or below the order/disorder temperature To/d for the polymer.
Ordering and defect annihilation may be combined into a single annealing process or a plurality of processes may be used in order to provide a layer of self-assembled polymer such as BCP, having an ordered pattern of domains of differing chemical type (of domains of different block types).
In order to transfer a pattern, such as a device architecture or topology, from the self-assembled polymer layer into the substrate upon which the self-assembled polymer is deposited, typically a first domain type will be removed by so-called breakthrough etching to provide a pattern of a second domain type on the surface of the substrate with the substrate laid bare between the features of the second domain type. A pattern having parallel cylindrical phase domains can be etched using a dry etching or reactive ion etching technique. A pattern having lamellar phase domains can utilize a wet etching technique in addition to or as an alternative to those suitable for the etching of parallel cylindrical phase domains.
Following the breakthrough etching, the pattern may be transferred by so-called transfer etching using an etching means which is resisted by the second domain type and so forms recesses in the substrate surface where the surface has been laid bare.
While the discussion herein focuses on graphoepitaxy guiding templates, it will be appreciated that a guiding template may involve a technology other than, or in addition to, graphoepitaxy. For example, the guiding template may be a chemoepitaxy template involving chemical surface modification of the guiding template, wherein the chemical modification facilitates guiding of the self-assembly. For example, in a chemoepitaxy guiding template for aligning a di-block copolymer having A and B blocks, where A is hydrophilic and B is hydrophobic in nature, the surface of the template may comprise one or more hydrophobic strips with a neutral orientation base between the hydrophobic features. The strips have a similar function as the one of more hydrophobic walls of a graphoepitaxy guiding template.
Optical Proximity Correction (OPC)
As an example, OPC addresses the fact that the final size and placement of one or more features of an image of a design pattern projected on a substrate will not be identical to, or simply depend only on, the size and placement of the one or more features of the design pattern at the patterning device. It is noted that the terms “mask”, “reticle”, “patterning device” are utilized interchangeably herein. Furthermore, masks and reticles can be broadly termed “patterning devices.” Further, a person skilled in the art will recognize that, especially in the context of lithography simulation and optimization, the term “mask,” “patterning device” and “design pattern” can be used interchangeably, as in lithography simulation and optimization, a physical patterning device is not necessarily used but a design pattern can be used to represent a physical patterning device. For small feature sizes and/or high feature densities present on some design pattern, the position of a particular edge of a given feature may be influenced to a certain extent by the presence or absence of other adjacent features. These proximity effects arise from minute amounts of radiation coupled from one feature to another and/or non-geometrical optical effects such as diffraction and interference. Similarly, proximity effects may arise from diffusion and other chemical effects during post-exposure bake (PEB), resist development, and etching that generally follow lithography.
In order to help ensure that the projected image of the design pattern is in accordance with requirements of a given target design, proximity effects should be predicted and compensated for, using sophisticated numerical models, corrections or pre-distortions of the design pattern. The article “Full-Chip Lithography Simulation and Design Analysis—How OPC Is Changing IC Design”, C. Spence, Proc. SPIE, Vol. 5751, pp 1-14 (2005) provides an overview of current “model-based” optical proximity correction processes. In a typical high-end design almost every feature of the design pattern has some modification in order to achieve high fidelity of the projected image to the target design. These modifications may include shifting or biasing of edge positions or line widths as well as application of “assist” features that are intended to assist projection of other features.
Application of model-based OPC to a target design involves good process models and considerable computational resources, given the many millions of features typically present in, for example, a chip design. However, applying OPC is generally not an “exact science”, but an empirical, iterative process that does not always compensate for all possible proximity effects. Therefore, the effect of OPC, e.g., design patterns after application of OPC and any other resolution enhancement techniques (RET), should be verified by design inspection, e.g. intensive full-chip simulation using calibrated numerical process models, in order to reduce or minimize the possibility of a design flaw being built into the patterning device pattern. This is driven by the enormous cost of making high-end patterning devices, which run in the multi-million dollar range, as well as by the impact on turn-around time by reworking or repairing actual patterning devices once they have been manufactured.
Both OPC and full-chip RET verification may be based on numerical modeling systems and methods as described, for example in, U.S. Pat. No. 7,003,758 and an article titled “Optimized Hardware and Software For Fast, Full Chip Simulation”, by Y. Cao et al., Proc. SPIE, Vol. 5754, 405 (2005).
One RET is related to adjustment of the global bias of the design pattern. The global bias is the difference between the features in the design pattern and the features intended to print on the substrate. For example, a circular feature of 25 nm diameter may be printed on the substrate by a 50 nm diameter feature in the design pattern or by a 20 nm diameter feature in the design pattern but with high dose.
In addition to optimization to design patterns or patterning devices (e.g., OPC), the illumination source may be optimized, either jointly with patterning device optimization or separately, in an effort to improve the overall lithography fidelity. The terms “illumination source” and “source” are used interchangeably in this document. Since the 1990s, many off-axis illumination sources, such as annular, quadrupole, and dipole, have been introduced, and have provided more freedom for OPC design, thereby improving the imaging results. Off-axis illumination is a proven way to resolve fine structures (i.e., target features). However, when compared to a traditional illumination source, an off-axis illumination source usually provides less radiation intensity for the aerial image (AI). Thus, it becomes desirable to attempt to optimize the illumination source to achieve the optimal balance between finer resolution and reduced radiation intensity.
Numerous illumination source optimization approaches can be found, for example, in an article by Rosenbluth et al., titled “Optimum Mask and Source Patterns to Print A Given Shape”, Journal of Microlithography, Microfabrication, Microsystems 1(1), pp. 13-20, (2002). The source is partitioned into several regions, each of which corresponds to a certain region of the pupil spectrum. Then, the source distribution is assumed to be uniform in each source region and the brightness of each region is optimized for process window. However, such an assumption that the source distribution is uniform in each source region is not always valid, and as a result the effectiveness of this approach suffers. In another example set forth in an article by Granik, titled “Source Optimization for Image Fidelity and Throughput”, Journal of Microlithography, Microfabrication, Microsystems 3(4), pp. 509-522, (2004), several existing source optimization approaches are overviewed and a method based on illuminator pixels is proposed that converts the source optimization problem into a series of non-negative least square optimizations. Though these methods have demonstrated some successes, they typically require multiple complicated iterations to converge. In addition, it may be difficult to determine the appropriate/optimal values for some extra parameters, such as γ in Granik's method, which dictates the trade-off between optimizing the source for substrate image fidelity and the smoothness requirement of the source.
For low k1 photolithography, optimization of both the source and patterning device is useful to help ensure a viable process window for projection of critical patterns. Some algorithms (e.g. Socha et. al. Proc. SPIE vol. 5853, 2005, p. 180) discretize illumination into independent source points and patterning device diffraction orders in the spatial frequency domain, and separately formulate a cost function (which is defined as a function of selected design variables) based on process window metrics such as exposure latitude which could be predicted by optical imaging models from source point intensities and patterning device diffraction orders. The term “design variables” as used herein comprises a set of parameters of a lithographic apparatus, for example, parameters a user of the lithographic apparatus can adjust. It should be appreciated that any characteristics of a lithographic process, including those of the source, the patterning device, the projection optics, and/or resist characteristics can be among the design variables in the optimization. The cost function is often a non-linear function of the design variables. Then standard optimization techniques are used to minimize the cost function.
Relatedly, the pressure of ever decreasing design rules have driven semiconductor chipmakers to move deeper into the low k1 lithography era with existing 193 nm ArF lithography. Lithography towards lower k1 puts heavy demands on RET, exposure tools, and the need for litho-friendly design. 1.35 ArF hyper numerical aperture (NA) exposure tools may be used in the future. To help ensure that a design pattern can be produced on the substrate with workable process window, source-patterning device optimization (referred to herein as source-mask optimization or SMO) is becoming a significant RET for 2×nm node.
A source and patterning device optimization method and system that allows for simultaneous optimization of the source and patterning device using a cost function without constraints and within a practicable amount of time is described in PCT Patent Application Publication No. WO 2010/059954, which is hereby incorporated by reference in its entirety.
Another source and mask optimization method and system that involves optimizing the source by adjusting pixels of the source is described in U.S. Patent Application Publication No. 2010/0315614, which is hereby incorporated by reference in its entirety.